It is known that vehicles of large size and to an increasing extent of medium and even small size are nowadays often epuipped with a power assistance steering to reduce driver effort and hence fatigue so as to thereby ensure improved safety.
Currently, a generally adopted power assistance steering apparatus is of a hydraulic type in which fluid is pumped to assist the movement of a steering rack. However in recent years an electrically driven power assistance steering apparatus such as described in U.S. Pat. No. 4,532,567 has been developed in an attempt to improve controllability and improve the "feel" of a steering wheel to a driver since many known power assistance steering apparatus tend to provide a "dead" feel to a driver which can be dangerous on wet or otherwise slippery road surfaces. Also an electric power assistance steering apparatus has the advantage that it is driven from the vehicle battery and so does not present a drain upon the vehicle engine since it will be readily understood by those skilled in the art that the hydraulic type of power assistance steering is normally driven by a belt from the engine crank shaft to pump the power assisting fluid.
In most of the known electrically driven assistance steering apparatus known to the applicants, an electric motor is employed as an electrical actuator because a large magnitude operating effort is required and the output of the electric motor is reduced by reduction gearing to provide the desired amount of steering assistance. Such an arrangement however leads to an extremely undesirable steering characteristic in that the feel of the steering at the steering wheel is improperly affected due to the inertia of the electric motor and by virtue of the significant frictional resistance produced by the reduction gear mechanism interposed between the electric motor and the steering rack.
In an attempt to improve the feel of the steering and to overcome the inertia of the electric motor and reduction gear mechanism a method of imparting a variable control characteristic to the electric motor is disclosed in Japanese Unexamined Patent Application Publication No. 76760/1980. However in this Japanese Publication the motor current command signal bears a predetermined functional relationship to the torque command derived from the steering wheel so that a compensating value is provided in proportion only to the torque command signal and such an arrangement has been found to involve instabliity in the amount of steering assistance. More specifically the current command signal is taught in the Japanese Publication to vary in a non-linear relationship, such as for example, a quadratic function relative to the torque command signal and the amount of compensation is thus not in direct proportion to the torque command. As a consequence, although the conpensating effect is significant when the torque command has a small magnitude, the compensating effect is reduced as the torque command is increased thereby giving rise to a tendency for the steering assistance to be degraded. Such an arrangement has been found to provide a power assistance steering apparatus in which the driver tends to overturn the steering wheel and he then has to apply a corrected steering movement to compensate for the over-steering operation that he has performed.